Thermoforming apparatus with web support means

ABSTRACT

Thermoforming apparatus wherein the opposite longitudinal edge portions of a sheet of thermoplastic material are gripped by an indexing conveyor which conveys the sheet through a heating oven wherein it is heated to forming temperature and through a thermoformer wherein the heated sheet is deformed into a mold cavity by pressure and/or vacuum, said apparatus being characterized in that at least the portion of the length of the heating oven adjacent the thermoformer has therein one or more perforated tubes which extend longitudinally between the gripped edges of the sheet with air being discharged upwardly through said tubes against the bottom surface of the sheet to form an air bearing support to inhibit sagging of the heated sheet between its gripped longitudinal edges. The apparatus herein is further characterized in that there is a heat interchange between the air and the sheet supporting tubes so that the latter are cooled to avoid possible overheating of the sheet thereabove while the air flowing through the tubes is heated so as not to have undue chilling effect on the regions of the sheet upon which the air jets impinge.

Unite States Patent Lynch [4 1 Feb. 18, 1975 THERMOFORMING APPARATUSWITH WEB SUPPORT MEANS John R. Lynch, Hartville, Ohio [73] Assignee: NRMCorporation, Akron, Ohio [22] Filed: Jan. 3, 1974 [2]] Appl. No.:430,408

[75] Inventor:

Primary Examiner-Robert L. Spicer, Jr. Attorney, Agent, or FirmDonnelly,Maky, Renner & Otto [57] ABSTRACT Thermoforming apparatus wherein theopposite longitudinal edge portions of a sheet of thermoplastic materialare gripped by an indexing conveyor which conveys the sheet through aheating oven wherein it is heated to forming temperature and through athermoformer wherein the heated sheet is deformed into a mold cavity bypressure and/or vacuum, said apparatus being characterized in that atleast the portion of the length of the heating oven adjacent thethermoformer has therein one or more perforated tubes which extendlongitudinally between the gripped edges of the sheet with air beingdischarged upwardly through said tubes against the bottom surface of thesheet to form an air bearing support to inhibit sagging of the heatedsheet between its gripped longitudinal edges. The apparatus herein isfurther characterized in that there is a heat interchange between theair and the sheet supporting tubes so that the latter are cooled toavoid possible overheating of the sheet thereabove while the air flowingthrough the tubesis heated so as not to have undue chilling effect onthe regions of the sheet upon which the air jets impinge.

6 Claims, 5 Drawing Figures THERMOFORMING APPARATUS WITH WEB SUPPORTMEANS BACKGROUND OF THE INVENTION tend through the thermoformer the moldcavities must be arranged to straddle said bands. Moreover, as said sagbands move through the oven toward the thermoformer they may reach oventemperature whereby the portions of the heated plastic sheet restingthereupon may be overheated with more pronounced scalloped formation andthinning of the sheet where it rests upon the overheated sag bands.

In lieu of sag bands traveling with the sheet through the oven andthermoformer it has been proposed to cause the sheet gripping chains totravel in a diverging path to compensate for the width-wise elongationof the sheet due to heating thereof. Yet another proposal heretofore hasbeen to include transverse sheet support strips in the oven over whichthe sheet is conveyed to inhibit sagging ofthe heated sheet in the ovenunder its own weight.

SUMMARY OF THE INVENTION In contradistinction to known thermoformingapparatus for inhibiting sagging of the heated sheet as it is conveyedto the thermoformer, it is herein proposed to provide stationarylongitudinally extending perforated tubes in the oven through which agaseous medium such as air is discharged upwardly to impinge on theunder surface ofthe sheet thus to form a gas bearing for low frictiontransport ofthe sheet through the oven and to inhibit sagging of theheated sheet.

It is a principal object of this invention to provide thermoformingapparatus of the character indicated in which the sag inhibiting sheetsupport means is in heat exchange relation with the gas bearing mediumsuch that the sheet support tubes will be at less than oven temperatureand such that the gaseous medium is heated so that the heated sheetsupported by the gas bearing is not unduly chilled.

Other objects and advantages will appear from the ensuing description.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a side elevation view ofathermoforming apparatus embodying the present invention;

FIG. 2 is a partial top view of the heating oven at the outlet zonethereof adjacent to the thermoformer illustrating the sag inhibitingtubes which provide an air bearing for indexing movement of the sheet asit is heated in the oven and advanced into the thermoformer;

FIG. 3 is a cross-section view on enlarged scale taken substantiallyalong the line 33, FIG. 2 illustrating the sheet gripping means whichgrips a longitudinal edge portion of the sheet and illustrating one ofthe sag inhibiting tubes which extend longitudinally of the sheet andparallel to the side edges thereof to provide a gas bearing to supportthe sheet for transport through the oven to the thermoformer with lowfriction;

FIG. 4 is an elevation view as viewed along the line 44, FIG. 2; and

FIG. 5 is a much enlarged top plan view as viewed along the line 5-5,FIG. 4 illustrating the series perforations through the top of the saginhibiting tube through which a gaseous medium such as air flowsupwardly to impinge on the bottom surface of the heated sheet as it istransported through the oven thus to form a gas bearing of low frictioncharacteristics.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT As shown in the drawingand especially FIG. I thereof, the thermoforming apparatus comprises abase structure 1 on which is supported a thermoformer 2 with a heatingoven 3 on one side thereof and having on the other side thereof a drivemeans 4 for moving the top run of a pair of sheet gripping and conveyingchains 5 length-wise through the heating oven 3 and the thermoformer 2in stepwise manner.

Each chain 5 is of the pin chain type having a series of pins 6 thereonwhich at the righthand end 7 of the apparatus as viewed in FIG. I areoperative to puncture the opposite edges ofthe sheet S to be heated andthermoformed and to convey the sheet S thus gripped by the pin chains 5longitudinally through the heating oven 3 and thermoformer 2. Thepunctured edge portions of the sheet S are supported by longitudinallyextending cooled guide rails 8 which are mounted on channel members 9 onopposite sides of the oven 3. The oven 3 has a series of electricheating elements 10 or the like disposed in spaced relation above andbelow the sheet 5 which is transported through the oven 3 by said pinchains 5. For a detailed description of the pin chain conveyor 5, theheating oven 3, the thermoformer 2, and the drive means 4 for the pinchain conveyor 5 reference may be had to the aforesaid Herbener US. Pat.No. 3,632,272. has

opposite sides of the oven 3 have angular heat shields 11 and 12 securedthereto which are spaced apart as shown to accommodate the edge portionsof the gripped sheet and to shield such edge portions from the intenseheat in the oven 3. Extending lengthwise in the oven 3, especially atthe end thereof adjacent the thermoformer 2 whereat the sheet S reachesforming temperature, are sag inhibiting means which comprise one or morelongitudinally extending tubes 14 having perforations 15 through the topthrough which air under pressure is introduced through a filter andpressure regulator l6 and air manifold 17 with flexible hose 18connected to the ends of the tubes 14- which extend laterally from theoven 3. The tubes 14 are supported at spaced intervals by adjustablebrackets 19 bolted to the lower heat shields l2 and the ends of thetubes 14 remote from the thermoformer end of the oven 3 are slopeddownwardly so as not to be engaged by the sheet S and at that portion ofthe oven 3 the sheet S is not yet heated to extent such as to requiresupport against sag ging. Such sloping portions of the sheet supportingtubes 14 have a useful function in that they are yet disposed in theoven 3 and hence serve to preheat the air prior to its being dischargedfrom the series of orifices 15 to impinge on the bottom surface ofthesheet S disposed thereon.

By way of illustrative example, the sag inhibiting tubes 14 may be Ainch diameter stainless steel tubes with holes of 0.01 3 to 0.015 inchdiameter spaced apart A inch and with an air pressure of, say, 15 psi inthe manifold 17 the air jets directed upwardly from the tubes 14impinges on the bottom surface of the sheet S thereabove to form an airbearing for ease of transport of the sheet S through the oven 3 and,moreover, although the tubes 14 otherwise would reach oven temperature,the air flow therethrough is in heat exchange relation with the tubes 14so that the tubes are cooled to less than oven temperature and by thesame token the interchange of heat heats the air so that its impingementon the bottom surface of the sheet S does not re sult in undue chillingof portions of the sheet directly above the tubes 14.

The brackets 19 aforesaid include screw means 20 and slots 21 whichenable desired spacing of the sag inhibiting tubes 14 with respect tothe gripped edge portions of the sheet S and it will be apparent thatdepending upon the width of the sheet S and its gage one, two, or moresag inhibiting tubes 14 may be employed. It is to be understood that thesag inhibiting tubes 14 may extend throughout the length of the oven 3or a major portion thereof but it has been found that in an oven of sayfoot length, the straight horizontal portion of the sag inhibiting tubes14 may be of say 4 foot length with perforations extending for somewhatless than 5 feet, partly extending into the sloping portion which is outof contact with the sheet S. Accordingly, as the line of travel of thesheet S approaches the horizontally extending portion of each saginhibiting tube 14, the upwardly directed jets of air progressivelyoffer increasing support to the sheet S which is of progressivelyincreasing temperature so that by the time that the sheet S reaches thejuncture of the sloping portion and the horizontal portion of each tube14 the sheet S will be supported by an air film between its bottomsurface and the upper surface of each sag inhibiting tube 14 to providefor low friction transport of the sheet S. By reason of the heatinterchange between the tube 14 and the air, the tubes 14 are cooled toa temperature less than oven temperature and by the same token the airis heated so as not to impose an undue chilling effect on the lines ofthe sheet S which are disposed directly above the sag inhibiting tubes14. Moreover, the slop ing and lateral portions of each tube beingdisposed in the oven serve to preheat the air.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. Thermoforming apparatus comprising an elongated heating oven into oneend of which a thermoplastic web is introduced for transporttherethrough for heating said web to forming temperature; a thermoformeradjacent the other end of said oven into which the heated web istransported for forming therein; a conveyor operative to grip theopposite longitudinal edge portions of said web to convey said webthrough said oven and thermoformer for respectively heating said web toforming temperature and forming the web; sag inhibiting web supportmeans in said oven extending longitudinally from such other end of saidoven toward such one end and disposed underneath said web to support thelatter between its longitudinal edge portions, said means comprising atleast one elongated tube laterally spaced between said edge portions andhaving a series of openings through the wall thereof juxtaposed to thebottom surface of the web; and a fluid pressure source to supply gaseousmedium into said tube for flow upwardly through said openings to impingeon the web thereabove thus to define a gas bearing to facilitatetransport of the web through said oven while inhibiting sagging thereof.

2. The apparatus of claim 1 wherein said gaseous medium is introducedinto said tube at a temperature substantially less than oven temperatureand which gaseous medium is in heat exchange relation with said tube tocool the tube to less than oven temperature and to heat the airdischarged through said openings so as not to have an undue chillingeffect on the portions of the web upon which said air jets impinge.

3. The apparatus of claim 1 wherein said oven has heat shields onopposite sides thereof forming longitudinally extending gapstherebetween through which the edge portions of the web pass to protectthe gripped edge portions from the high temperature in the oven; andwherein there are longitudinally spaced apart tube supporting bracketson the lower one of said shields to support said tube beneath thetraveling web.

4. The apparatus of claim 3 wherein said brackets are laterallyadjustable to predeterminately space said tube from the lateral edges ofsaid web.

5. The apparatus of claim 1 wherein said tube has a laterally extendingportion extending through one side of said oven in spaced relationbeneath the traveling web and a longitudinally extending portion whichslopes upwardly toward the web supporting portion thereof thus toprovide an additional length of tube within said oven for preheating ofsaid gaseous medium before reaching said series of openings.

6. The apparatus of claim 5 wherein said openings begin in the slopingportion of the tube to provide anprogressively increasing impingementeffect as the sloping portion approaches the web supporting portion. l ll

1. Thermoforming apparatus comprising an elongated heating oven into oneend of which a thermoplastic web is introduced for transporttherethrough for heating said web to forming temperature; a thermoformeradjacent the other end of said oven into which the heated web istransported for forming therein; a conveyor operative to grip theopposite longitudinal edge portions of said web to convey said webthrough said oven and thermoformer for respectively heating said web toforming temperature and forming the web; sag inhibiting web supportmeans in said oven extending longitudinally from such other end of saidoven toward such one end and disposed underneath said web to support thelatter between its longitudinal edge portions, said means comprising atleast one elongated tube laterally spaced between said edge portions andhaving a series of openings through the wall thereof juxtaposed to thebottom surface of the web; and a fluid pressure source to supply gaseousmedium into said tube for flow upwardly through said openings to impingeon the web thereabove thus to define a gas bearing to facilitatetransport of the web through said oven while inhibiting sagging thereof.2. The apparatus of claim 1 Wherein said gaseous medium is introducedinto said tube at a temperature substantially less than oven temperatureand which gaseous medium is in heat exchange relation with said tube tocool the tube to less than oven temperature and to heat the airdischarged through said openings so as not to have an undue chillingeffect on the portions of the web upon which said air jets impinge. 3.The apparatus of claim 1 wherein said oven has heat shields on oppositesides thereof forming longitudinally extending gaps therebetween throughwhich the edge portions of the web pass to protect the gripped edgeportions from the high temperature in the oven; and wherein there arelongitudinally spaced apart tube supporting brackets on the lower one ofsaid shields to support said tube beneath the traveling web.
 4. Theapparatus of claim 3 wherein said brackets are laterally adjustable topredeterminately space said tube from the lateral edges of said web. 5.The apparatus of claim 1 wherein said tube has a laterally extendingportion extending through one side of said oven in spaced relationbeneath the traveling web and a longitudinally extending portion whichslopes upwardly toward the web supporting portion thereof thus toprovide an additional length of tube within said oven for preheating ofsaid gaseous medium before reaching said series of openings.
 6. Theapparatus of claim 5 wherein said openings begin in the sloping portionof the tube to provide anprogressively increasing impingement effect asthe sloping portion approaches the web supporting portion.